1. Field of the Invention
The present invention relates to minute structures, devices having minute structures, and manufacturing methods thereof. In particular, the present invention relates to a nanoscale structure, a device using the nanoscale structure, and manufacturing methods thereof. The nanoscale structure of the present invention provided with alumina nanoholes can be widely used as a functional material and as a structural material for electronic devices and micro devices. The present invention can be particularly applied to quantum effect devices, electrochemical sensors, biosensors, magnetic memory devices, magnetic devices, light-emitting devices, photonic devices, solar cells, and the like.
2. Description of the Related Art
Since the movement of electrons is constrained when thin films, thin wires, and dots of a metal and a semiconductor are smaller than the distinctive length (such as mean free path or spin scattering length) thereof, they may exhibit unique electrical, optical, and chemical characteristics in some cases. From the point of view mentioned above, minute structures (nanostructures) smaller than 100 nanometers (nm), have attracted significant attention as a functional material.
As a method for manufacturing the nanostructure, semiconductor processing techniques, such as fine pattern drawing techniques including photolithography, electron-beam lithography, x-ray lithography, and the like, may be mentioned as examples.
In addition to the method mentioned above, attempts to realize a novel nanostructure using a structure having naturally formed regularity, i.e., a structure which is formed in a self-organizing manner, have been made. Depending on the minute structure used as the base structure, the method mentioned above has the possibility of producing a fine and unique structure superior to that obtained by the conventional method. As a result, much research has been initiated in this area. As an example of a unique structure formed in a self-organizing manner, an anodized alumina film may be considered (refer to R. C. Furneaux et al, “The formation of controlled-porosity membranes from anodically oxidized aluminium” Nature, vol. 337, p. 147 (1989), and the like).
When an aluminum (Al) plate is anodized in an acidic electrolyte, a porous oxide film is formed. FIG. 9 is a schematic cross-sectional view showing a nanostructure having a porous alumina nanohole layer 11 formed on an Al plate 31 by anodization thereof. As shown in FIG. 9, the characteristic of the anodized alumina film is a unique geometric structure thereof, in which extremely fine pores (nanoholes) 12 having diameters 2r of a few nm to a few hundreds of nm are disposed at a few tens of nm to a few hundreds of nm intervals 2R therebetween. The nanoholes 12 have high aspect ratios. In addition, between the anodized alumina nanoholes 12 and the Al substrate, barrier layers (aluminum oxide layers) 22 exist.
Focusing on the unique geometric structure of alumina nanoholes formed by anodization, various applications thereof have been implemented. Application of anodized films to films using abrasion resistance and insulating properties thereof and application to filters obtained by peeling anodized films are as examples. In addition, by filling the nanoholes with a metal, a semiconductor, or the like or by using a replica of nanoholes (by using nanoholes as a mold), various applications to coloration, a magnetic recording medium, an electroluminescent (EL) light-emitting device, an electrochromic device, an optical device, a solar cell, a gas sensor, and the like have been pursued. Furthermore, applications in various fields have been anticipated, such as a quantum effect device including a quantum thin wire, a metal insulator metal (MIM) device, a molecular sensor using nanoholes as a chemical reaction field, and the like (see Masuda, “Metal nanohole array having high regularity formed of anodized alumina”, KOTAIBUTSURI (Solid State Physics) 31, vol. 31, no. 5, pp. 493-499 (1996), Japanese Patent Laid-Open No. 11-200090).
Concerning the formation of a nanostructure by using semiconductor processing techniques described above, there are problems in that the production yield is low and the apparatuses therefor are costly. Hence, a simple and reproducible manufacturing method has been desired. In view of the points described above, the method using self-organization, in particular, the method of anodizing Al, has advantages in that a nanostructure can be easily and controllably formed.
Since the nanostructure shown in FIG. 9 is only formed on an Al plate surface, the application and the shape thereof are limited to some extent. For example, since the melting point of Al is 660° C., nanoholes formed on the surface thereof cannot be subjected to heating to or above the above-mentioned temperature. Consequently, in order to use nanoholes as a functional material in various fields, a novel technique has been awaited in which an anodized alumina film can be formed on a substrate having a higher melting point without impairing the unique geometric structure of the anodized alumina or cracking at higher temperatures.
When applications of the minute structure described above to devices are considered, it is important that the pores be filled with a functional material and that a plurality of pores be selectively filled with a functional material.